Wood is treated in sulfate cooking process by white liquor containing NaOH and Na.sub.2 S, whereby lignin dissolves and cellulose fibers are released. The mixture of cellulose fibers (pulp) and cooking chemicals is then treated with water, resulting in the generation of black liquor. The black liquor is next concentrated by evaporation in an evaporation plant. The black liquor thereby concentrated is cornbusted in a kraft recovery boiler, and the chemical melt thereby produced and containing mainly Na.sub.2 S and Na.sub.2 CO.sub.3, is dissolved in water, resulting in the generation of green liquor. The green liquor is next causticized by caustic lime (CaO) to white liquor containing NaOH and Na.sub.2 S, and lime sludge mainly comprising CaCO.sub.3. This white liquor is transferred back to the digester, and the lime sludge is calcinated in a lime sludge reburning kiln to caustic lime which is recycled back into the causticizing stage.
in a sulfate pulp mill sulfur emissions are generated mainly in the kraft recovery boiler, the evaporation plant and the digester house. In order to decrease the environmental impact of the pulp mill the sulfur emissions should be minimized. It has been found that an increase in the dry solids content of black liquor causes a decrease in the sulfur emissions of the kraft recovery boiler. On the other hand, the sulfur content of green liquor increases as a consequence thereof and thus also results in an increase in the sulfidity of the white liquor as well as the sulfur content of tile black liquor. As a result, the overall sulfur emissions of an evaporation plant will increase as a consequence of the continuous increase of sulfur in the black liquor as described above. The instant invention is directed to overcoming this problem by removing the sulfur containing gas prior to or during tile evaporation process, or immediately prior to combustion of the black liquor (after evaporation thereof).
U.S. Pat. No. 4,929,307 shows that the viscosity of tile black liquor can be decreased by heating the liquor to a temperature higher than its cooking temperature. Consequently, it is possible to evaporate tile black liquor to a higher dry solids content, while also decreasing the sulfur emissions of the kraft recovery boiler. Also U.S. Pat. No. 2,711,430 discloses that heating of black liquor causes tile release of organic sulfur compounds.
Surprisingly, it has been discovered that the above mentioned phenomena can be utilized in a completely new manner. It is thus an object of the instant invention to utilize the above-mentioned phenomena to control the sulfidity of a suifate pulp mill.